In appliances such as these, special precautions have to be taken in order to minimize the loads on the mains power supply system caused when these appliances are switched on. For this purpose, a known power supply having inrush current limiting SBG and having a voltage regulator SRE is illustrated in FIG. 1. An input AC voltage Uw is in this case supplied, after rectification in a rectifier GLB, as a pulsed DC voltage Ug via a current limiting circuit SBG to an energy-storage capacitor C, the voltage across which is Uc, and which is connected in parallel with the input of a voltage regulator SRE or some other load. Ua denotes the output voltage of the regulator SRE.
The energy-storage capacitor C is used for bridging short-term failures of the input voltage Ug, which is in the form of a DC voltage or, in particular, an unsmoothed voltage which is produced at the output of a rectifier GLB. The energy-storage capacitor C generally has a high capacitance and, when the power supply is switched on, it must first of all be charged up, with the current limiting circuit SBG preventing excessively high inrush current surge values during this charging process.
FIG. 2 shows a further known power supply with inrush current limiting, in which the load current drawn from the regulator SRE in this case does not flow via the current limiter SBG, in order to reduce the power loss. For the purposes of this application, the differences between FIG. 1 and FIG. 2 are, however, irrelevant.
As soon as the capacitor C has been charged up, the current limiting must assume a low resistance in order that the currents flowing do not cause any further losses and, if necessary, the capacitor C can emit its energy with a low internal impedance to the load. Such active current limiting is designed in a characteristic manner, for example with current sources.
When a capacitor is charged up via such current limiting, virtually the same amount of energy which is absorbed by the capacitor is converted into heat in the transistor which is used for current limiting. The power loss in the current limiting element is directly proportional to the voltage difference between the input voltage and the capacitor voltage, assuming that the limiting current is set such that it is constant. The power loss in the limiting element thus fluctuates between zero and a maximum value within one half-cycle of the input voltage Ug. The power loss which occurs is particularly critical in power supplies which have to emit a relatively high current during normal operation since, as is self-evident, the limiting current must be higher than the maximum operating current. Owing to the short-term load on the limiting transistor which is normally used, this has had to be greatly derated, since it is necessary to design it for the pulse power loss at the voltage maximum in conjunction with the transient thermal impedance.